1. Field of the Invention
The present invention relates to the papermaking and related arts. More specifically, the present invention relates to papermaker""s fabrics, namely the forming, press and dryer fabrics, also known collectively as paper machine clothing, on which paper is manufactured on a paper machine. In addition, the present invention may find application in other industrial settings, where industrial belts are used to dewater a material, such as in pulp washing and thickening and in sludge dewatering.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, on a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which cause the cellulosic fibers in the web to adhere to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speed. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Operating, as they do, in the form of endless loops on paper machines, papermaker""s fabrics, and particularly their inner surfaces, are susceptible to abrasive wear. Much of this wear results from contact with stationery components of the paper machine. Ultimately, many fabrics must be removed from paper machines when the wear caused by such moving contact has reduced the thickness of the fabric, at least in some locations, to the point where it is weakened or has lost some quality or characteristic desired by papermakers in its particular application.
Normally, wear is monitored using a thickness gauge. However, it is difficult to measure the thickness of a papermaker""s fabric more than a foot or two in from its edges with such a gauge, especially when the fabric is running on a paper machine.
Clearly, a means for monitoring wear on a papermaker""s fabric, and particularly at any point on its inner and outer surfaces, even when the paper machine is operating, would be very helpful to those in the papermaking industry. The present invention provides such a means to the industry.
Accordingly, the present invention is an industrial fabric of the variety used in papermaking and related industries, and in other industries where a fabric is used in dewatering some material. The industrial fabric comprises machine-direction (MD) yarns and cross-machine direction (CD) yarns, which may be interwoven with one another to give the industrial fabric a woven structure.
At least some, and possibly all, of either the MD yarns or the CD yarns, or of both the MD and CD yarns, are sheath/core yarns. The sheath/core yarns comprise a core yarn surrounded by a sheath, wherein the core yarn and the sheath are visually distinguishable by the naked eye from one another, such as by color. This enables wear on a surface of the industrial fabric to be monitored visually during the operating life thereof, as abrasion gradually wears away the sheath on exposed portions of the sheath/core yarns, particularly surface knuckles, revealing the core yarns, perhaps by their differing color.
The present invention will now be described in more complete detail with frequent reference being made to the drawings identified below.